1. Technical Field
The present invention relates in general to a fuel additive that is a combustion catalyst, and in particular to an additive containing an over-based magnesium compound combined with a soluble iron compound for which the median particle size in the additive is less than about 0.01 micrometers.
1. Description of the Prior Art
Energy can be produced by the combustion of fuels in combustion equipment, such fuels including but not limited to fossil fuels such as liquid petroleum, solid hydrocarbon fuels, and other fuel products, including wood fuels. A common concern is the reduction of particulate emissions from such combustion equipment. Another common concern is increasing fuel efficiency in such equipment. The use of combustion catalysts has been shown to generate results with regard to both of these concerns.
One example of a fuel used in such combustion equipment is petroleum fuel. Refining of petroleum consists principally of separating fractions of the oil according to distillation fractions. Following removal of gas, the first boiling fraction is No. 1 fuel or naphtha. The next fraction, No. 2 fuel, is up to the limit of atmospheric distillation. This fraction includes gasoline fuel, kerosene and jet fuels. No. 4 fuel is the portion distilled under vacuum. No. 6 fuel is the residual fuel left behind following vacuum distillation. (No. 3 and No. 5 are usually mixtures.) The naphtha fraction contains a wide range of molecular structures with low-molecular weight. Some of these structures yield high octane numbers and other structures low octane. During most of the 20th century, a large amount of the portion with low octane number could be used with octane enhancing products.
Many chemical compounds have been used over the past century to improve octane number as engine efficiencies have increased with higher compression ratios. The use, and subsequent banning, of lead has been known for a long time. Tetraethyl lead showed a positive effect on octane and a profoundly negative effect on the environment. Catalytic cracking will accomplish the same result of increasing octane number, but with an enormous fixed cost in equipment. Catalytic cracking of residual fuel has been used to increase the volume of high-octane naphtha grade fuels. These processes are extremely efficient leading to 40% or more of the barrel of crude available for use as gasoline.
In addition to tetraethyl lead, several elements are known to have combustion catalyst characteristics. Examples, in addition to lead, are manganese, iron, copper, cerium, calcium and barium. Each of these elements has advantages and disadvantages in particular applications. In the past, iron has been evaluated, mainly in the form of bis-cyclopentadienyl iron (0) or ferrocene. Drawbacks of ferrocene include limited solubility in gasoline, toxicity, and expense as an additive. Other iron compounds in oil soluble form or as dispersions have been evaluated with similar drawbacks of toxicity and expense. Iron products typically increase the Real Octane Number or RON by about 2 units. However, iron compounds typically react with sulfur in the naphtha feed stock to form iron sulfide precipitate, which is undesirable.
Another commonly used additive in gasoline is MTBE. While this compound boosts octane levels significantly, the compound is thought to be carcinogenic. Also, it mixes easily with water which is hazardous should there be a leak. Gasoline containing MTBE leaking from an underground tank at a gas station could potentially leach into groundwater and contaminate wells. As a result of the believed negative potential of MTBE on the environment, ethanol is also being evaluated as a gasoline additive to boost octane.
The effects of various metals listed above are known to improve combustion in boilers and combustion turbines and metals but these metals are known to vary ash quality. In addition to iron, useful first row transition metals from the periodic table include manganese and copper. Also, various alkaline earth metals (barium, calcium) and others such as cerium, platinum and palladium have been tested. Manganese is most widely used as a combustion catalyst in boilers with residual oil that often contains fuel contaminants, such as vanadium. Platinum and palladium, generally found in catalytic converters, are quite expensive. Manganese, when used alone, also forms low melting deposits and negates effects of magnesium on control of vanadium/sodium/calcium/potassium deposits. Iron catalyzes sulfur trioxide formation from sulfur dioxide increasing “cold end” corrosion (exhaust area) and sulfuric acid “rain” problems. Copper is less effective than either iron or manganese. Calcium forms tenacious deposits with other contaminant metals. Barium forms toxic salts. Cerium is not considered effective because of its higher elemental weight. These metals have been demonstrated to reduce smoke by no more than 50% at concentrations of up to about 50 PPM on a weight/weight basis by Environmental Protection Agency Test Method 5 (EPC M-5). While these metals have been evaluated in turbines and boilers, octane number is not at issue in this environment. Stability of the metal molecules is also not at issue and therefore not tested in boiler and turbines.
In addition to the industry goal of improved combustion efficiency, smoke emissions reduction is also a concern. U.S. Provisional Patent Application Ser. No. 60/373,249 describes a method for reducing smoke and particulate emissions from high speed (>1,000 rpm), high-compression reciprocating spark-ignited engines, such as gasoline engines.
Marine engines, which are substantially different in design and fuel type from spark-ignited engines, have been the subject of research on additives to reduce smoke emissions. Dispersion-type manganese (Mn) and iron (Fe) compounds have been used to reduce smoke emissions in low-speed (150-400 rpm) marine diesel engines. However, these compounds produce solid material in the gaseous phase. Marine diesel engines are capable of tolerating such gaseous phase solid materials because such engines have large piston and bore size tolerances as compared with higher speed gasoline engines. Moreover, marine diesel engines consume large amounts of crankcase oil in the combustion process, which may help to reduce solid material accumulation. Medium (450-1,000 rpm) and high speed (>1,000 rpm) engines cannot tolerate high levels of contamination of crankcase oil from combustion products. However, dispersion-type manganese and iron compounds have not been shown to have any synergistic relationship for combustion catalysis.
Over-based magnesium (Mg) compounds are known in the art for converting trace metal contaminants into high melting compounds and reducing deposits in combustion turbine engines operated by liquid petroleum fuels containing trace metal contaminants such as vanadium, lead, sodium, potassium and calcium. These contaminants form low melting point corrosive deposits on hot metal parts in reciprocating engines, such as low-speed marine diesel engines. Magnesium is known to form high-melting salts with vanadium, sodium and other fuel contaminants. As a result, over-based magnesium compounds are used alone as fuel additives for compression-ignited reciprocating engines to reduce the effects of these contaminants. For example, an over-based magnesium compound has been used alone in a Wartsilla V32 18 cylinder 8 MW stationary diesel engine, to alleviate the effects of deposits and corrosion from the residual oil fuel used.
While the iron additives and the magnesium additives have been effective, a fuel additive that includes an improved combustion catalyst to further reduce smoke and particulate emissions from boilers, engines, and other equipment operating on fossil and other fuels would be advantageous.